A charge pump is a DC-DC converter that receives an input voltage VIN and provides a different output voltage VOUT. The charge pump comprises one or more energy storage elements (usually capacitors) and switches that selectively apply voltages to the energy storage elements.
In the act of converting an input voltage to a lower or higher output voltage there are some energy losses. A charge pump has an efficiency that can be characterized by an equivalent resistance (RE). RE is known to vary for a given load depending on the switching frequency of the switches that are used to operate the charge pump, and on the passive components used for the charge-pump.